Some macroinvertebrates (bugs) graze on organic films (including diatoms) that grow on the streambed. For example: snails and some mayflies
Other macroinvertebrates filter drifting food particles from the stream current, including small fragments of decomposing leaf litter, tiny invertebrates and even wastes of other invertebrates (Example caddisflies).
Some are predators that catch live prey. Example: Dragonflies and damselflies.